Cartridge case comparison is based on the observation that microscopic firearm imperfections (such as those on a breech-face) can be transferred to a fired casing. The ability to certify two casings as highly similar is therefore a function of both the ability to capture a high-resolution three dimensional measurement of each casing and the ability to identify and match relevant structural features between two casings.
For over 90 years, firearm induced toolmarks have been manually compared using light-microscopy. This approach is time consuming and lacks an interpretable quantitative measure of similarity resulting in an increasing number of courtroom admissibility challenges. With this manual approach, expert examiners would typically consider micron-scale geometry toolmarks for qualitative comparisons, but due physical limitations, quantitative measurements of the toolmark dimensions were unpractical at the micron-scale.
Initial commercial systems, introduced in the 1980s and 90s, combined traditional 2D light microscopy with a digital camera and software for image comparison and database search. When the system is used in a database search, an image of the query object is compared to a stored library of previously collected images. Hits are ranked by match score, and presented in a rank list. A forensics expert sequentially considers each match and when possible, may take both pieces of evidence to a light microscope for manual comparison and confirmation. Unfortunately, current 2D systems suffer from several disadvantages that often result in low match accuracy due to image quality and correlation accuracy which are significantly affected by lighting conditions. Surface features visible under one lighting condition may be virtually invisible with a small lighting change. A further limitation of the traditional 2D methods is the sole reliance on length and width parameters of the toolmarks. This approach omits important geometric height data from consideration.
Researchers are now exploring second generation technology capable of capturing three-dimensional images of toolmarks. Several technologies have been considered, including: focus-variation microscopy, confocal microscopy, point laser profilometry, and scanning interferometry.
The listed techniques introduce crucial geometric height parameters that increase accuracy and consistency over traditional manual and 2D methods of analysis. Focus-variation and confocal microscopy derive geometric height information based on the focal plane of the detector. Point laser profilometry and scanning interferometry determine geometric height variations based on interference patterns created along the optical path of a detector.
Of these, confocal microscopy and focus-variation microscopy were recently identified as the most promising. However, the nature of confocal microscopy presents limitations when dealing with steep slopes, artifacts of surface reflectivity, acquisition speed, and cost. While only a small number of labs may be able to afford confocal-based 3D imaging machines, there are many laboratories and research facilities that would benefit from a lower-cost three-dimensional imaging solution.
The current state of cartridge case comparison also suffers in common methods of analyzing casings. One common method is to determine a cross-correlation between the corresponding pixel values of two casing images. The underlying assumption is that once two images are normalized, they should contain the same structural features and thus similar pixel values. Current cross-correlation based determinations suffer from a few shortcomings.
First and most importantly, non-informative features can adversely affect the match score. The entire breech-face is noisy and contains both informative and non-informative structural features. That is, while some of the structural features are similar between the two surfaces, other features are not; because the cross-correlation typically considers the entire masked surface, both informative and non-informative regions are compared and the quality of the match can be negatively affected.
Second, the multiple pre-processing steps have the potential to eliminate relevant information in each image.
Finally, imaging artifacts and shadows can adversely affect the match score. Researchers at NIST recently extended the cross-correlation method into their Contiguous Matching Cells (CMC) approach. The CMC method divides the measured surface into a series of patches (or cells) and compares each independently. This reduces the influence of non-matching regions when comparing two surfaces and performs better than straight cross-correlation based methods.
It is worth noting that 2D image comparison algorithms have been known to incur problems in translating to the comparison of 3D images.
To further complicate the error prone approach, existing approaches to providing match analysis simply provide a match score and do not localize the specific features identified as a basis for the score. As a result this “black box” approach raises issues concerning the arbitrary nature of the score and doubt as to the overall reliability.
A method to visually present the basis of the match score as well as providing real-time light manipulation is needed in the art as a tool to aid forensics experts to provide reliable and consistent analysis of micron scale toolmarks.
Another problematic issue facing the current field of forensic analysis is an inability to reproduce lighting conditions during manual analysis. Furthermore, it is not feasible for forensic experts to be able to manipulate light positioning at micron level resolution. This substantially decreases the accuracy and consistency of casing matches between forensic experts as various lighting conditions produce different shading patterns that result in the interpretation of conflicting features, particularly at the micron level. Furthermore, issues arise with the difficulties in reproducing the analytical basis for match opinions. The field is in need of a tool that would provide a precise method of light manipulation at micron level resolution that can objectively track precise lighting parameters that can be reported and applied with reproducible accuracy.
Several recent reports, including two from the National Academy of Sciences, have called for additional research, development of new instrumentation and inventive methods to address the identified challenges and to usher in the next generation of forensic analysis.